The invention relates in general to the measurement of fluid pressure and, in particular, to a method and apparatus for converting an absolute fluid pressure to a differential fluid pressure.
In many fluidic circuits, it is necessary to convert the absolute pressure of a pressurized fluid to a differential pressure indicating the pressure of the pressurized fluid relative to a predetermined reference pressure. For example, in a pressure regulating circuit, a single pressure output signal from a rectifier may be used as a control signal to a differential circuit. If the desired reference pressure is available, the unknown pressure can be compared with the reference pressure in a differential amplifier.
However, in many applications, the desired reference signal is not available. In such a case, a passive circuit consisting of a parallel arrangement of an orifice and a capillary can be used to convert the unknown pressure into two flows which can be used as control pressure signals indicating the difference between the unknown pressure and a predetermined reference pressure. When the unknown pressure is correct, the flows are equal and no differential pressure occurs. When the pressure is low, more flow goes through the orifice giving rise to a differential signal in one direction. When the pressure is high, the flow is higher through the capillary giving rise to a differential signal in an opposite direction. Generally, pressure sensors for sensing an unknown fluid pressure inherently have a high input impedance. In this known arrangement, due to this high input impedance, the orifice will always contain a high content of linear or capillary features giving rise to a low sensitivity.